Sahih Bukhari Volume 002, Book 020, Hadith Number 188.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 20. Shortening The Prayers (At-Taqseer)
Narated By 'Abdullah bin 'Umar : I offered the prayer with the Prophet, Abu Bakr and 'Umar at Mina and it was of two Rakat. 'Uthman in the early days of his caliphate did the same, but later on he started praying the full prayer.